The present invention relates to an improved process for forming non-cyclic, aliphatic compounds having a multiplicity of primary amino groups and forming these compounds in high yields and selectivity from the corresponding polynitrile having at least 2 atoms between the cyano groups.
The hydrogenation of a wide variety of nitriles to their corresponding amines using conventional hydrogenation catalysts is well known. However, it is recognized that this mode of synthesis leaves much to be desired when the aim is to produce non-cyclic, aliphatic polyamines from polynitriles having an atomic structure capable of forming ring containing compounds. The presently known processes often provide the desired noncyclic products in low selectivity and yield, while the undesired cyclic compounds predominate. This is especially true with polynitriles such as nitrilotriacetonitrile (NTAN), iminoodiacetonitrile (IDAN) and ethylenediaminetetraacetonitrile (EDTN). Normally, the dominant products formed are cyclic polyamines. When one attempts to adjust the reaction conditions to those which may provide higher selectivity or yield of the noncyclic, aliphatic compound, one observes rapid inactivation of the catalyst materials used.
It is generally known that hydrogenation of nitriles can be accomplished by many modes such as by batch processing using a stirred autoclave or by continuous processing using a fixed bed reactor to contact a hydrogenation catalyst with a solution containing a nitrile. The reaction product is generally a mixture of primary, secondary and tertiary amines. The secondary and tertiary amines are by-product materials which are thought to occur by the reaction of some of the primary amine product with imine intermediate material (formed in the hydrogenation of the nitrile). In this process, the already formed primary amine reacts with imine intermediate to produce a secondary amine and, in turn, some of the secondary amine reacts with additional imine to produce a tertiary amine product.
When the starting nitrile has a multiplicity of cyano groups which are separated by an appropriate chain length of at least 2 atoms, the secondary and tertiary amine formation tends to be intramolecular to provide cyclic compounds as the dominant product. Thus, when a dinitrile, such as iminodiacetonitrile, is subjected to conventional hydrogenation, one forms the cyclic compound, piperazine, as the major material. For a trinitrile, such as nitrilotriacetonitrile, the difficulty of forming the corresponding linear aliphatic amine, tris(2-amino-ethyl) amine (TREN), increases geometrically. Thus, contacting of a polynitrile with a hydrogenation catalyst is a recognized route for producing cyclic polyamines.
The use of a batch reactor has been previously viewed as a process mode which promotes the formation of unwanted side products. The batch reactor normally used in this high pressure reaction is, by conventional design, a fixed vessel in which all of the reactants are initially charged into the reaction vessel and all of the primary amine product is retained within the vessel until the process is terminated. As the nitrile is converted to imine intermediate, there is an increased probability for it to react with previously formed amine contained within the vessel and be diverted into by-product formation . Thus, when polynitriles are converted into polyamines using a conventional batch reaction, one conventionally obtains large amounts of cyclic product as well as methylated secondary amine and condensation by-products. U.S. Pat. Nos. 3,565,957 and 3,733,325 teach that the yields of cyclic amine can be optimized by carrying out the reaction in the presence of large amounts of ammonia. By using a hydrogenation catalyst to increase the yield of linear product, one produces solid condensation material which inactivates the catalyst in a very short period of time. Further, U.S. Pat. No. 3,565,957 provides an example which attempts to teach the formation of linear amine, TREN, but clearly the product formed therein is not TREN as the product has a boiling point which is distinctly different from TREN. This reference thus exemplifies the difficulty in forming desired linear polyamine product by a batch mode. The short life of the catalyst as well as low selectivity and yield has caused this process to be deemed commercially unfeasible in providing linear polyamines.
To overcome the problems associated with synthesizing aliphatic polyamines from polynitriles in batch reactors, the use of a fixed bed (trickle-bed type) reactor has been suggested by Sherwin et al. in U.S. Pat. No. 4,721,811, which is assigned to the assignee of the present invention. While the fixed bed reactor can efficiently produce large quantities of the desired amines, it cannot match the cost-effectiveness of batch-type reactor vessels when relatively smaller output is contemplated. The fixed bed process is relatively complex to set up and generally must be customized for the particular reaction to be carried out. Once set up, it is quite inefficient to modify the fixed bed reactor to accommodate a different process and then re-modify the apparatus for the original process. Thus, the fixed bed reactor process is best suited for those situations where the apparatus can be dedicated to one single reaction process. Additionally, heat removal can be a special problem with such fixed bed reactors, for example in the reduction of nitro and nitrile compounds which have very high heats of hydrogenation.
The present process would enable one to provide an efficient discontinuous or batch process carried out in a batch reactor, e.g., a stirred autoclave reactor vessel, which once the desired quantity of product is produced, is easily and cost effectively capable of being changed over to use in another process.